This invention relates generally to a method of controlling open-angle glaucoma by surgery and more specifically to the use of a laser in surgery to reduce the thickness of the trabecular meshwork and tissue around Schlemm's Canal to increase filtration of the aqueous humor and thereby controlling the open-angle glaucoma.
Primary open angle glaucoma is a disease of unknown etiology also known as simple glaucoma, chronic glaucoma, glaucoma simplex, compensated glaucoma, and open angle glaucoma. The disease is characterized by the increase in intraocular pressure which results in atrophy of the optic nerve, visual field disturbances, and eventual blindness. In primary open angle glaucoma the anterior chamber angle appears normal to direct observation and the aqueous humor has free access to the trabecular meshwork. Secondary open-angle glaucoma can occur because of fibrovascular proliferation and the trabecular meshwork is abnormal. Obstruction of the trabecular meshwork prevents the filtration of aqueous humor with a resulting increase in intraocular pressure.
Open angle glaucoma can be treated medically or surgically. The preferred treatment is medical and is directed to increasing the outflow of aqueous humor from the anterior chamber of the eyeball or by decreasing the secretion of aqueous humor, or both.
Primary open angle glaucoma can be treated with drugs such as timolol, an adrenergic receptor antagonist; pilocarpine, a cholinergic stimulating drug; echothiophate iodide, a cholinesterace antagonist; epinephrine, an alpha and beta agonist; and acetazolamide, a carbonic anhydrase inhibitor.
There are problems inherent with medical treatment. Miotic drugs, such as pilocarpine, may aggravate the visual loss caused by incipient cataract or may induce painful ciliary muscle spasms. Epinephrine may be irritating to the eye. Echothiophate iodide has a myriad of adverse effects, drug interactions and contraindications, as does acetazolamide. Timolol is contraindicated in patients with asthma and other pulmonary diseases.
If the glaucoma cannot be controlled by drugs and there is progress in the associated visual field disturbances and optic nerve atrophy, surgery is indicated.
The main surgical procedure used in the treatment of open angle glaucoma in which the trabecular meshwork is visible is laser trabeculoplasty, commonly using an argon laser. The eye is anesthetized and the trabecular meshwork is visualized through a gonioprism. The laser energy is applied ab externo to photocoagulate the trabecular meshwork. The laser can reduce the circumference of the trabecular ring by heat induced shrinkage of the collagen of the sheet of trabecular tissue or by scar tissue contraction at the burn sites, forcing the ring to move toward the center of the anterior chamber, elevating the sheets and pulling open the trabecular spaces. Flow is increased through the trabecular spaces. The main complication in this type of surgery is a transient increase in intraocular pressure that may require medication to control. Control is usually achieved in about 85% of all patients, but most (75%) continue to require some medication. Control can be lost, however, with the passing of time and additional laser trabeculoplasty may not be effective.
In eyes where laser trabeculoplasty cannot be performed or where it fails to control pressure, a filtering operation is indicated. All previous filtering operations were based on the theory of creating fistula between the anterior chamber and the sub-conjunctival space through which aqueous humor can flow. Generally, the surgery was performed by scalpel. Trabeculectomy is the operation of choice. An operating microscope is used and a scleral flap is fashioned to expose the trabecular meshwork. A portion of the full thickness eye wall is excised and the scleral flap replaced. A filtering bleb often develops after surgery. This surgery is performed in an operating suite and complications can include excessively low intraocular pressure, flat anterior chamber, endophthalmitis, cataract, sympathetic ophthalmic and bullous keratopathy. Furthermore, a mechanical deep dissection down to Sclemm's canal has limited success because of the difficulty in judging the depth of dissection of the trabecular meshwork and the surgeon can inadvertently enter the anterior chamber with the surgical tool.
Attempts to remedy the drawbacks associated with traditional trabeculectomy have had some success. My U.S. Pat. No. 5,370,641 discloses a method of performing laser trabeculodissection wherein laser energy is applied to the corneo scleral bed under a scleral flap, tissue is removed until sufficient aqueous humor is coming through the ultra-thin remaining Schlemm's Canal and trabecular meshwork. The energy of the laser is absorbed by the outflowing aqueous humor thereby creating a self-regulating endpoint. The scleral flap is replaced with or without a suture.
I have discovered that there is one notable drawback to the method disclosed in U.S. Pat. No. 5,370, 641. Laser trabeculodissection is performed with an ultraviolet wavelength wherein the wavelength is less than 230 nanometers. The ultraviolet wavelength is unsuitable for fiberoptic transmission. I have determined that the preferred embodiment is an ultraviolet laser that employs a galvanometric scanning delivery system rather than a variable aperture (iris-diaphragm) delivery system. Examples of the galvanometric scanning delivery systems include the COMPAK-200 Mini-Excimer and the LASERHARMONIC, both manufactured by LaserSight, Inc. (Orlando, Fla.).
The advantages of a galvanometric scanning delivery system are rooted in the anatomy of the portion of the eye to be treated and also in the programmable features of the system. As shown in FIG. 1, the anatomy of the portion of the limbal area to be treated is characterized by a curvilinear shape with a radius of approximately 7.5 mm. The arc length and width of treatment in the corneal scleral bed partially is determined by the severity of the glaucoma; the more severe glaucoma requires a broader and longer arc of trabeculodissection. The width is limited by the fact that the average maximum with of the trabecular meshwork is less than 1 mm. The arc is limited by the circumference of the limbal area around the eye.
Moreover, the trabecular meshwork is covered by an uneven amount of comeoscleral tissue. Specifically, the anterior-most portion near the scleral septum id deeper and thinner that is the posterior portion neat the scleral spur (iris root). The latter portion is more superficial and thicker. In addition, the ablation rate of comeal tissue is different from scleral tissue.
The goal of laser trabecular dissection is to achieve as wide an area (anteroposterior) as possible of partial thickness dissection over the trabecular meshwork, especially the posterior portion, sufficiently deep to allow for adequate aqueous drainage but not too deep that the dissection enters the anterior chamber,
A galvanometric scanning delivery system (GSS) is ideally suited to meet the above objectives. Specifically, the laser is programmed for a low, but suprathreshold, fluence, typically less than 120 mJ/cm.sup.2 to reduce the risk of full thickness penetration. The pulse frequency is set, typically at 60 to 300 Hertz, to achieve as rapid ablation as possible so as to reduce the potential for slow filtration that could absorb the laser energy and mask the laser effects. The GSS also allows selection of small spot size in the range to 100 to 300 microns.
Variable aperture delivery systems cannot be preprogrammed to achieve an appropriate ablation profile. The lack of a homogenous energy profile can create hot and cold spots and increase the risk of full perforation. Moreover, the attendant acoustic shockwave promotes premature drainage of aqueous that interferes with the ablation.
Furthermore, prior art laser trabeculodissection has an endpoint. When filtration begins, the aqueous humor absorbs the laser energy and masks the laser's effect. It would be beneficial, therefore, to use a method of surgery that allows filtration to proceed at one ablation site without interfering with the laser energy at a subsequent site.